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Life and Health 



person will judge the two warm disks to be about the same weight, 

 or lighter, than the single cold disk. 



Experiment 133. To illustrate the influence of excitation of one 

 sense organ upon the other sense organs. On listening to the ticking 

 of a watch, the ticking sounds feebler or louder on looking at a light 

 through glasses of different colors. 



If the finger be placed in cold or warm water the temperature 

 appears to rise when a red glass is held in front of the eyes. 



Experiment 134. To show the movements of the iris. Look 

 through a pin hole in a card at a uniform white surface, as the 

 white shade of an ordinary reading lamp. With the right eye 

 look through the pin hole, the left eye being closed. Note the size 

 of the (slightly dull) circular visual field. Open the left eye, the field 

 becomes brighter and smaller (contraction of pupil) ; close the left 

 eye, after an appreciable time, the field (now slightly dull) is seen 

 gradually to expand. One can thus see and observe the rate of 

 movements of his own iris. 



- Experiment 135. To show the blind spot. The left eye being 

 shut, let the right eye be fixed on the cross as in Fig. 146. When 



FIG. 146. 



the book is held at arm's length both cross and round spot will be 

 visible ; but if the book be brought to about eight inches from the eye, 

 the gaze being kept steadily upon the cross, the round spot will at 

 first disappear, but as the book is brought still nearer both cross and 

 round spot will again be seen. 



Experiment 136. To illustrate the 

 duration of retinal impressions. On 

 a circular white disk, about halfway 

 between the center and circumference, 

 fix a small, black, oblong disk, and rap- 

 idly rotate it by means of a rotating 



^. P " CD ' S * S - wheel. There appears a ring of gray 

 (The disk A, when rotated rap- , . , L Al . 



idly, gives an even gray tint on the black > showm g that the im P rCS ' 

 in \ sion on the retina lasts a certain time. 



